Study Material — 1 PYQs (2020–2020) · Concept Notes · Shortcuts
RRB Group D Simple Interest is a frequently tested subtopic — 1 previous year questions from 2020–2020 papers are included below with concept notes, key rules and shortcut tricks.
1 questions from actual RRB Group D papers · all shown free · click option to reveal solution
Exam Q 12020Previous Year Pattern
A sum of money becomes ₹4,800 after 4 years at a simple interest rate of 5% per annum. What is the principal amount?
Concept Notes
Simple Interest— Rules & Concept
Core ConceptRead this first — the foundation of the topic
Simple Interest is the extra money paid on borrowed money or earned on invested money. It is calculated only on the original amount (called Principal) for a specific time period at a fixed rate. Core Concept: Simple Interest remains constant every year. If you borrow Rs. 1000 at 10% simple interest, you pay Rs. 100 every year as interest. The principal amount never changes in calculations.
Formula BlockMemorise — at least one formula appears in every paper
Block:
Simple Interest (SI) = (P × R × T) / 100
Amount = Principal + Simple Interest
Principal (P) = (SI × 100) / (R × T)
Rate (R) = (SI × 100) / (P × T)
Time (T) = (SI × 100) / (P × R)
Exam PatternsWhat examiners ask — read before attempting PYQs
SSC CGL consistently asks 2-3 questions on Simple Interest. Common question types include finding SI when P, R, T are given, calculating time or rate when other values are known, and comparing simple vs compound interest scenarios.
Master Shortcut #1 - Quick SI Calculation:
For easy percentages, use direct multiplication:
- 10% of any amount = Amount/10
- 5% of any amount = Amount/20
- 20% of any amount = Amount/5
Worked ExampleSolve this step-by-step before moving on
1
Step 1
Identify P = 8000, R = 12%, T = 3 years
2
Step 2
Apply formula SI = (P × R × T) / 100
3
Step 3
SI = (8000 × 12 × 3) / 100
4
Step 4
SI = 288000 / 100 = Rs. 2880
5
Step 5
Amount = 8000 + 2880 = Rs. 10,880
Shortcut #2 - Time-Rate Relationship:
If rate doubles, time becomes half for same SI.
If time doubles, rate becomes half for same SI.
This helps eliminate wrong options quickly.
Worked Example 2:
Question: At what rate will Rs. 5000 amount to Rs. 6500 in 4 years at simple interest?
1
Step 1
Amount = 6500, Principal = 5000
2
Step 2
SI = Amount - Principal = 6500 - 5000 = Rs. 1500
3
Step 3
Using R = (SI × 100) / (P × T)
4
Step 4
R = (1500 × 100) / (5000 × 4)
5
Step 5
R = 150000 / 20000 = 7.5%
Shortcut #3 - Percentage Method:
When principal becomes 'n' times in 't' years:
Rate = [(n-1) × 100] / t
Example: If money doubles (n=2) in 10 years, Rate = (2-1) × 100/10 = 10%
Most Common Trap: Students often confuse the time unit. If rate is per annum but time is given in months, convert months to years by dividing by 12. Always match the time unit with the rate unit. This single mistake costs many marks in SSC CGL.
Another frequent error is adding interest multiple times. Remember, in simple interest, you add interest only once to get the final amount, unlike compound interest where interest compounds.
Key Points to Remember
Simple Interest formula: SI = (P × R × T) / 100 where P=Principal, R=Rate, T=Time
Amount = Principal + Simple Interest (add only once, not yearly)
SI remains constant every year unlike compound interest which grows
Quick calculation: 10% SI = Principal/10, 20% SI = Principal/5
If money becomes n times in t years, Rate = [(n-1) × 100] / t
Time and rate are inversely proportional for same SI amount
Always convert time units to match rate units (months to years or vice versa)
Principal can be found using: P = (SI × 100) / (R × T)
Rate doubles means time halves for same SI amount earned
Common trap: Never compound the interest in simple interest problems
Exam-Specific Tips
Standard SI formula uses division by 100, never by 1000 or other numbers
When rate is given per annum, time must be in years for direct calculation
If principal doubles, the rate-time product always equals 100
SI for 2 years at 10% rate equals 20% of principal amount
Principal formula derivation: P = (SI × 100) / (R × T)
Rate formula: R = (SI × 100) / (P × T) gives percentage value
Time formula: T = (SI × 100) / (P × R) gives time in rate's unit
Amount formula: A = P + SI = P[1 + (R × T)/100]
Practice MCQs
Simple Interest — Practice Questions
6graded MCQs · easy to hard · full solution & trap analysis
Rajesh borrowed ₹8,000 from a bank at a simple interest rate of 12% per annum. How much interest will he pay after 3 years?
Practice 2medium
Rajesh borrowed ₹5,000 at 8% per annum simple interest. After how many years will the simple interest amount to ₹1,600?
Practice 3medium
The simple interest on a sum for 6 years at 9% per annum is ₹2,700. What will be the simple interest on the same sum for 4 years at 6% per annum?
Practice 4medium
Rajesh borrowed ₹8,000 from a bank at a simple interest rate of 12% per annum. After 3 years, he repaid the loan. How much total amount did Rajesh pay to the bank?
Practice 5medium
A person invests ₹8,000 at 6% per annum simple interest. How much more interest will he earn if he keeps the money invested for 5 years instead of 3 years?
Practice 6hard
Rajesh borrowed ₹8,000 at a simple interest rate of 12% per annum. After some time, he repaid ₹10,240. However, the lender made an error and charged him interest for 2 years more than the actual loan period. If the error had not been made, how much less would Rajesh have paid?
60-Second Revision — Simple Interest
Formula: SI = (P × R × T) / 100, Amount = P + SI
Remember: Always match time units with rate units before calculation
Shortcut: 10% rate means SI = Principal × Time ÷ 10
Trap: Never add interest multiple times in simple interest
Quick check: If money doubles in t years, rate = 100/t percent
Inverse relation: Rate doubles, time halves for same SI
Convert months to years by dividing by 12 when rate is per annum